A plank of mass 10 kg and a block of mass 2 kg are placed on a horizontal plane as shown in the figure. There is no friction between plane and plank. The coefficient of friction between block and plank is 0.5 .A force of 60 N is applied on plank horizontally. In first 2 s the work done by friction on the block is

Options

A. `-100 J`
B. 100 J
C. zero
D. 200 J

Answer

B

Explanation

(b) Maximum acceleration of 2 kg block due to friction can be `mu g or 5 ms^(-2)`.
Combined acceleration, if both move together with same acceleration would be,
`a=(60)/(12)=5ms^(-2)`
Since, both accelerations are equal, upper block will move with acceleration `5ms^(-2)` due to friction.
In first two seconds, `s=(1)/(2)at^(2)=(1)/(2)xx5xx4=10m`
and force of friction, f=ma=10 N
`:. " " W_(f)=fs cos0^(@)=100J`